148 Chapter 5 and ammonium concentrations ranged from 200 µM to 1200 µM (Supplementary Figure 1A). Sulfide concentrations dropped below the detection limit within two hours after their addition during both spikes and the long-term exposure period (Supplementary Figure 1B). 0 40 80 120 160 Activity relative to control (%) ∼24 h ∼5 days Batch Bioreactor 0.25 0.5 T0 0.5 T3 0.5 Na2S (mM) * * * * control control Figure 2. Methane oxidation potential in the microcosm experiment (batch) or bioreactor experiments is expressed as activity relative to control (either batch or bioreactor). The batch experiment includes n=2 for control and n=3 for experimental conditions. Relative activity was calculated either by comparing the first 24 h, or during the entire monitoring period of 5 days. Methane oxidation potential was calculated by cumulative 45CO 2 / 44CO 2 normalized by dry weight (g). Significant differences (t-test) of the different incubations compared to the control are indicated with an asterisk (P<0.05). For the microcosm experiment, the bar indicates standard deviation (n=2-3). The average cumulative 45CO 2/ 44CO 2 g-1 day-1 for the bioreactor and batch controls were: 2.04-02 and 1.21-5 ± 1.00-6, respectively. Our experiments showed a strong inhibition of ~ 45% (bioreactor) and ~ 12% (batch) of methane oxidation activity of the anaerobic consortium with the methanotroph “Ca. Methanoperedens” after a 0.5 mM sulfide spike, which recovered to ~ 74% and ~ 77% after 5 days, respectively (Figure 2). These inhibitory thresholds contrast with those reported for brackish ANME S-AOM, where ~ 1 mM sulfide inhibited 50% of AOM activity (Dalcin Martins et al., 2024, Chapter 2). However, for the measured S-AOM sulfide-inhibition, the in situ sulfide was not converted and remained
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